Watches, in which the dial and the hands are illuminated by means of an ultraviolet light source, are already known in the prior art. Various zones of the dial and hands are formed or covered by a substance that reacts to ultraviolet light and reflects this in the form of visible light back through the glass, thus allowing said zones to be seen in a dark environment.
For example, the patent document WO 2004/034153 describes a watch comprising a light-emitting diode or LED that emits light in the ultraviolet (UV) spectrum and is arranged on the inner periphery of the watch case between the dial and the glass of the watch. This LED is secured in a recess by a transparent resin that allows ultraviolet rays to pass through. By reacting with the ultraviolet rays, a so-called “luminous” material mixed with the transparent resin emits visible light. The dial and hands, also provided with this “luminous” material, are illuminated by the LED with ultraviolet and visible light at the same time, thus making it easier to read the time in a dark environment.
This type of watch, which combines a source of ultraviolet light with a luminous material requires a power supply, e.g. in the form of electric power with a battery. The power transformed by the LED into ultraviolet light will be largely dispersed with part of the ultraviolet rays escaping through the glass of the watch in the direction of the eyes of the user, which poses a risk for this person. In addition, since the respective surface of the hands and the index marks of the dial is relatively small, only a limited amount of ultraviolet light will be transformed into visible light by the “luminous” material, and this is unsatisfactory in terms of efficiency and illumination.
The patent document JP 2003-248445 describes a watch fitted with an ultraviolet light source associated with a waveguide provided under the dial of the watch. The latter is perforated by holes that may or may not be filled with a luminescent material that reacts with the ultraviolet rays in order to emit visible light, but can also allow the ultraviolet light to pass through the dial. Such a solution requires the use of a particular dial that is dedicated specifically to this application and is superposed on the waveguide, which poses a disadvantage both in terms of the method of assembly that must be adapted and in terms of the resulting thickness of the timepiece, which is then substantially greater.
Moreover, although the use of a waveguide in this type of watch allows losses to be slightly reduced by concentrating the ultraviolet light on the luminescent materials, the losses caused still remain significant, since the ultraviolet light that has not been transformed into visible light also partially escapes through the glass of the watch in the direction of the eyes of the user.
In all the cases outlined above the lighting devices comprise an LED that does not enable the time to be read in an optimum manner in a dark environment.